Certain organs, such as the liver, are particularly immunologically active and may mount stronger immune responses against gene therapy vectors. Taken together, these technical and economic considerations complicate which delivery method is best for gene therapies. During the 1990s, researchers recognized the potential of a modified, non-pathogenic form of lentivirus as gene therapy vectors. So let’s say we wanted to design a gene therapy to introduce the short sleeper genotype, DEC2, into humans. The impressive clinical successes achieved across existing methods suggest that the gene therapy field is only just getting started.